Aeromonas hydrophila is an important pathogen that has caused major loss in the aquaculture industry (Shotts et al., 1972; Olivier et al., 1981; Esteve et al., 1995) and attempts have been made to develop an effective vaccine (Lamers et al., 1985; Baba et al., 1988b; Leung et al., 1997; Rahman and Kawai, 2000). The effects of a number of inactivated whole cell (WC) vaccines have been reported and an increase in serum antibody levels against A. hydrophila was observed in common carp immersed in a preparation of heat inactivated A. hydrophila (Lamers et al., 1985). Kusuda et al. (1987) also found an increase in the concentration of total serum proteins, when common carp were immunised with formalin killed A. hydrophila. Rainbow trout immunised by injection, immersion and oral administration of killed A. hydrophila, have been shown to produce antibodies in their serum, bile, skin and gut mucus, and skin and muscle extracts (Loghothetis and Austin, 1994). A polyvalent vaccine containing heat killed WC and formalin inactivated extra cellular products (ECP) of A. hydrophila has also been tested in two Indian major carp species (rohu and mrigal), but it failed to protect the fish against bacterial challenge (Chandran et al., 2002a).
Bacteria biofilms grown on the surfaces of nutrient flakes (chitin) have been used as oral administered heat inactivated biofilm vaccines against A. hydrophila, and have been reported to elicit a protective response in Indian major carp (catla and rohu) and common carp (Azad et al., 1999). The biofilm vaccines have been found to be retained for longer than free cell vaccines in the tissues of gut, spleen and kidney in Indian major carp (Azad et al., 2000a). Post A. hydrophila challenge, Catfish, fed with biofilm vaccines showed significantly higher serum agglutinating antibody titres and relative percentage survival (RPS) values compared to those fed with free cells (Nayak et al. 2004b). When grown as a biofilm, it was noted that there was a change in A. hydrophila antigenicity (Asha et al., 2004). Specifically, the authors found that the S-layer proteins were lost and the Lipopolysaccharide (LPS) of the bacteria contained an additional high molecular weight band. Asha et al. (2004) suggested that this high molecular weight LPS band might elicit a protective immune response when the biofilm was administrated as an oral vaccine.
Considerable interest has been shown in bacterial OMP vaccines. Rahman and Kawai (2000) found that the OMPs of A. hydrophila elicited protection against an A. hydrophila challenge, and suggested that a vaccine based on selected OMP antigens may be effective. Munn (1994) suggested outer membrane components such as LPS could represent protective vaccine candidates for Gram-negative bacteria, while Dooley et al. (1986) reported that LPS of A. hydrophila posses highly immunogenic O polysaccharide chains of homogeneous length, and these were conserved both morphologically and antigenically in virulent isolates. The role of LPS in protection was also shown, in common carp vaccinated with crude LPS (Baba et al., 1988b). This protection appears to be based on cellular immunity rather than humoral immunity (Baba et al., 1988a). Similarly, Loghothetis and Austin (1996b) reported that LPS could be a major antigenic component of A. hydrophila. 
Live WC cell vaccines have also been found to increase antibody responses in fish (Loghothetis and Austin, 1994). Other live vaccines, such as live attenuated (mutant) vaccines have also been explored for A. hydrophila. For example, growth-deficient mutants of A. hydrophila have been found to be promising live vaccine candidates in fish (Leung et al., 1997). An AroA mutant A. hydrophila strain was also investigated and found to be protective in rainbow trout (Moral et al., 1998). The vaccine was also found to elicit significant protection against A. salmonicida (Vivas et al., 2004b). Vivas et al. (2004c) suggested that this live aroA attenuated vaccine had a high level of safety compared with normal strains as it has a lower potential to survive in water. Other live vaccines have also been investigated, for example Catfish, Clarias batrachus immunised with plasmid free A. hydrophila mutants, showed an increased survival rate following challenge with wild bacteria compared to the control group (Majumdar et al., 2006). Mutant strains of A. hydrophila with a highly attenuated exoenzyme were also shown to confer protection in swordtail fish, Xiphophorus helleri (Liu and Bi, 2006).
Although all the vaccines reported have shown varying degrees of increased immunity and protection, no commercial vaccine is available for A. hydrophila (Loghothetis and Austin, 1996b; Rahman and Kawai, 2000; Fang et al., 2004; Vivas et al., 2005). Over 90 established or provisional serogroups within the genus Aeromonas have been described, and the heterologous nature of A. hydrophila, both biochemically and serologically are still the greatest concern for developing an effective vaccine against A. hydrophila (Sakazaki and Shimada 1984; Stevenson, 1988; Khashe et al., 1996; Newman, 1993; Janda et al., 1994b; Leung et al., 1995).
It is amongst the objects of the present invention to obviate and/or mitigate at least one of the aforementioned disadvantages.
It is a further object to provide a vaccine against A. hydrophila, which desirably exhibits broad protection to many serogroups of A. hydrophila. 